Three Types of Diodes

8/12/2019 Three Types of Diodes

1/32

Planar Diode Fabrication

ELEC 3908, Physical Electronics, Lecture 5


2/32

ELEC 3908, Physical Electronics:

Planar Diode FabricationPage 5-2

Lecture Outline

Last lecture described a number of processing techniques

used to fabricate integrated circuits

This lecture will show how those techniques are usedtogether, some many times, in fabricating three integrated

diode structures

As more complex structures are considered, the level ofdetail in the descriptions will be reduced


3/32



Diode Types Considered

Fabrication of three types of diodes examined:

Substrate Diode: simple pn-junction fabricated from a single

counterdoped region in the substrate

Well Diode: slightly more complicated structure with a deeper

region of counter doping and a highly doped diffusion

Epitaxial Diode: More complicated processing using an epitaxial

layer, but offers the best performance


4/32



Substrate Diode - Nitride Protection

First step is to deposit a layer of silicon nitride (Si3N4) over the wafer

surface

Would normally be done using chemical vapor deposition (CVD)


5/32



Substrate Diode - Photoresist Coating

Surface (top of nitride layer) then coated with photoresist (PR)


6/32



Substrate Diode - Exposure

Surface of PR is then exposed to UV radiation through a mask created

from geometry information supplied by the designer


7/32



Substrate Diode - Development of Photoresist

Photoresist is then developed chemically

A negative photoresist remains where it was exposed to UV


8/32


9/32



Substrate Diode - Finished Nitride Etch

When the nitride etching is complete, all of the nitride layer outside the

remaining area of photoresist has been removed

Both nitride and photoresist remain in the exposed area


10/32



Substrate Diode - Photoresist Removal

The photoresist still covering the remaining nitride area is now

removed


11/32



Substrate Diode - Thermal Oxidation

A layer of silicon dioxide is grown using thermal oxidation

The oxide is prevented from growing in the area covered by silicon

nitride - this is the purpose of the nitride layer


12/32


13/32



Substrate Diode - Implantation

An implantation (ion implantation or diffusion) is now done to create a

counterdoped region which will form one side of thepn-junction

The oxide absorbs the dopant outside of the active area, preventing

dopant from penetrating into the substrate anywhere but the active area


14/32



Substrate Diode - Surface Metal Patterning

Metal is now deposited over the entire wafer surface

Another series of patterning steps is used, along with another mask, to

remove metal everywhere except the contact to the diode and wherever

else the connection is made


15/32



Substrate Diode - Substrate Connection

Metal is deposited on the backside of the wafer to form the other

connection

Note that all substrate diodes share a common (substrate) connection


16/32



Well Diode

Two problems with the substrate diode:

Current flows through the entire thickness of the substrate (500 -

1000 m) to reach the back contact

Substrate is common for all diodes on the chip, diodes all have a

common connection

Better solution is to use a well diode, which is formed in a

region of opposite doping (counterdoped) to the substrate,and a heavily doped region of the same type as the

substrate

Eliminates long current path through the substrate, andallows two independent terminals, since well is isolated

from the substrate


17/32



Well Diode - Nitride Deposition, Thermal Oxidation

A layer of nitride is deposited and patterned so that it exists on where

the active area (including the well) is to be formed

Thermal oxidation used to form an oxide layer


18/32



Well Diode - Well implant

A deep implantation is done to create the well - a counterdoped region

which will be one side of the diode


19/32


20/32



Well Diode - Diode Diffusion

The other side of the pn-junction structure is formed with a heavy

implant into the well


21/32



Well Diode - Isolation Oxide

A layer of silicon dioxide is deposited on the surface (thermal

oxidation would grow into the existing diffusion structure)

This layer is required because the two contacts to the diode are both at

the surface, hence an isolation layer is required to prevent shorting


22/32



Well Diode - Contact Cuts and Metallization

Contact cuts are etched through the isolation oxide to the diffusions

using a full series of patterning steps

Metal is deposited on the surface and patterned for interconnections

Provided the well to substrate junction is reverse biased, the well diodeis isolated from the substrate, and hence from other devices


23/32



Epitaxial Diode

Well diode is an improvement over the substrate diode, but

current flow is lateral so the exact performance is hard to

predict

Best solution, but with corresponding process complexity,

is the epitaxial diode, fabricated on an epitaxial layer of

silicon


24/32


25/32



Epitaxial Diode - Buried Layer Formation

Using a photolithography step (and an associated mask) a window is

formed in the oxide and a heavy n-type implant performed to create a

highly doped n-type (n+) region called theburied layer

The oxide is then removed using a selective etching step The result is a heavy n+ doping which will form the back connection to

the diode


26/32



Epitaxial Diode - Epitaxial Deposition

The next step is to use epitaxy to deposit a layer of high quality

crystalline silicon called the epi layeron the wafer surface

Some diffusion of dopant from the n+ region occurs into the epi layer

Another series of photolithography steps is used to form a maskingoxide over the region which will become the active diode area


27/32



Epitaxial Diode - Isolation Implants

A heavy p-type (p+) implant is then used to form regions extending

right through the epi and into the substrate (lateral diffusion also

results in extension under masking oxide)

These isolation regions electrically isolate the device from all othersfabricated in the epi layer


28/32



Epitaxial Diode - n+ andp+ Implants

Oxide is deposited and patterned to open a window for an n+ doping

which will form a contact through the n-type epi layer down to the

buried layer

Oxide is again deposited and patterned to produce an opening for aheavyp-type implant which will form the other side of thepn-junction

with the epi layer


29/32


30/32



Epitaxial Diode - Current Flow

The active diode area is only a small portion of the epitaxial structure

Current flow in the epi diode is through the active area, along the

buried layer and up and out the n+ contact diffusion

Benefit is well controlled current flow path

Also forms a major portion of the structure of an integrated BJT


31/32



Summary of Diode Structures

Three diode structures examined

Substrate: simple, but poor performance

Well: better, and getting more complicated

Epi: best, but most complicated

lightly doped substratep-

p+ p+n+p

+

n+buried layer

n-epitaxial layer

p-substrate

n-well

p+n+

Al

substrate thickness not to scale


32/32



Lecture Summary

The use of the basic processing techniques from lecture 4

in creating three diode structures was discussed

Note that many of the techniques are performed over andover as successive features are created

The substrate diode is simple but suffers from at least one

disadvantage all substrate diodes have one terminal

connected together

The well diode is an improvement, but has primarily lateral

flow, which can be difficult to characterize

The epi diode gives the best performance, but is much

more complex to fabricate than the first two

Documents

Three Types of Diodes